1. Field of the Invention
The present invention broadly pertains to a urea-formaldehyde condensate suitable for producing a stable urea-formaldehyde liquid fertilizer suspension having a slowly releasable source of nitrogen. This invention also pertains to a process of preparing a fertilizer suspension from the urea-formaldehyde condensate. This invention particularly relates to a stepwise process for preparing the fertilizer suspension, which process optimally controls the polymerization reactions between urea and formaldehyde to yield a product ideally suited for use as fertilizer.
2. Description of Prior Art
Fertilizers based on polymerization products of urea and formaldehyde are well known. By providing a slow release of nitrogen, these fertilizers enhance plant growth while minimizing root and foliage damage, e.g., burning, caused by excessive amounts of readily available nitrogen. Urea-formaldehyde fertilizers are commercially available as both solids and liquid suspensions, and the numerous patents describing the preparation and use of such fertilizers are generally familiar to those skilled in the art. While initial development concentrated on the solid ureaform fertilizers, liquid fertilizers have generally become more popular because they can be handled more easily and can be more uniformly distributed at the point of use.
An early disclosure of a liquid fertilizer suspension based on urea and formaldehyde is Waters et al, U.S. Pat. No. 3,096,168. Waters reacts a molar excess of urea with formaldehyde in aqueous phosphoric acid, and terminates the reaction by the addition of an ammonia source. A stable suspension is reportedly obtained by violently agitating the resulting slurry. Unfortunately, the activity index of the fertilizer suspension produced by this procedure tends to be quite low.
Formaini U.S. Pat. No. 3,677,736 describes a process for preparing a urea-formaldehyde fertilizer suspension wherein a molar excess of urea is initially reacted under alkaline conditions with formaldehyde in the presence of ammonia, followed by acidification and further reaction at an elevated temperature. Upon neutralization, a fertilizer suspension is obtained. While the resulting product has a much improved activity index relative to Waters' product, the suspension typically lacks the desired storage stability needed for commercial use.
Moore, U.S. Pat. No. 4,033,745 prepares a stable non-burning liquid fertilizer suspension containing highly polymerized urea-formaldehyde by initially reacting a molar excess of urea with formaldehyde at an elevated temperature and under a strongly basic condition. The reaction mixture is thereafter cooled and slowly acidified under vigorous agitation. The ensuing polymerization exotherm raises the reaction mixture temperature to about 70.degree. C., where it is then maintained until the mixture attains a desired consistency. The reaction is subsequently terminated by the addition of ammonia. At this point, other fertilizer nutrients, e.g., phosphorus and potassium; water-soluble sugar; and an aliphatic alcohol are added to the suspension with agitation to yield the final product. The alcohol reportedly helps improve the suspension's storability by inhibiting polymer growth, while the added sugar promotes microbial activity at the application site. Moore also indicates that the sugar helps to improve suspension stability. Since this procedure permits a high degree of polymerization, the activity index of the fertilizer product tends to be quite low.
Moore U.S. Pat. No. 4,145,207 describes an alternative procedure wherein an elevated temperature aqueous alkaline mixture of urea and formaldehyde is rapidly acidified with polyphosphoric acid. Rapid acidification reportedly initiates the formation of a large number of polymer chains. By thereafter carefully controlling the reaction mixture pH in accordance with the mixture's temperature rise, the polymer chains purportedly grow uniformly. With rapid cooling, a water-soluble sugar and an aliphatic alcohol are then added to the acidic mixture. Reaction between urea-formaldehyde polymers and added aliphatic alcohol reportedly reduce polymer growth and disproportionation during storage. The suspension is then stabilized using a gelled attapulgite clay.
Sartoretto U.S. Pat. No. 4,298,512 describes an acid catalyzed process for preparing a liquid fertilizer suspension wherein undesirable polymer growth is regulated by including a higher aldehyde, i.e., one having from 2 to 4 carbon atoms, in the initial urea and formaldehyde reaction mixture. The suspension produced is subsequently stabilized by the addition of a thickener-suspending agent, e.g., a vegetable gum such as xanthan gum.
Besides a controlled release of nitrogen and stability, another desirable property of liquid fertilizer suspensions, at least for certain commercial applications such as residential turf fertilization, is a low staining character. Staining refers to the tendency of dried fertilizer suspension particles to remain for protracted periods on a substrate, e.g., grass blades, driveways, sidewalks, etc. The dried suspension appears as a white deposit and homeowners generally find this deposit unsightly. It is known that staining is inversely related to particle size, i.e., smaller particles stain more than larger particles. Consequently, particle size considerations for a stable fertilizer suspension typically tend to conflict with those needed for a non-staining suspension.
It is an object of the present invention to provide a process for preparing a stable urea-formaldehyde condensate suitable for producing a urea-formaldehyde fertilizer suspension having a slowly releasable source of nitrogen.
It is also an object of the present invention to provide a process for preparing a urea-formaldehyde fertilizer from the condensate having a slowly releasable nitrogen source for enhancing plant growth.
It is another object of this invention to provide a process for preparing a stable, urea-formaldehyde liquid fertilizer suspension.
It is a further object of this invention to provide a process for producing a liquid fertilizer suspension having a relatively high activity index, i.e., above about 40.
It is still another object of this invention to provide a process for preparing a urea-formaldehyde fertilizer suspension which does not cause excessive foliage damage.
It is yet a further object of this invention to provide a process useful for producing a non-staining, stable urea-formaldehyde fertilizer suspension.